If the signal from HBO, Cinemax, etc is DD 5.1 then that is the mode your receiver will automatically use. If you have DPLIIx you can convert 5.1 to 6.1 or 7.1.
I find that most of my premium channels and most of the others as well are DD 2.0. I find that DPLII Music mode with the center width set to 4 is slightly better than DPLII Movie Mode (spreads the center a bit to the l/r speakers making the soundstage wider). PLII Music/Movie also works well for analog signals.
Occassionally analog signals don't sound quite right using the PLII decoder and I switch to either stereo or all channel stereo for those. I personally don't care for Neo:6, although it is not that different than PLII.
Thats what I do, but in the end you have to experiment a bit to determine what you prefer.